Electron discharge amplifier



March 4, 1941.

H. P. THOMAS ELECTRON DISCHARGE AMPLIFIER Filed March 23,,311940 CARR/El? voune:

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MODULATION 8 I; I SOURCE Invntor':

Henry P? Thomas, b9 W His Attorney.

Patented Mar. 4, 1941' UNITED STATES OFHCE Henry P. Thomas, Schenectady, N. Y., assignor to General Electric Company, a ccrporation of New York Application March 23, 1940', Serial No. 325,586

6 Claims.

My invention relates to an electron discharge amplifier and has particular application to an amplifier of oscillatory currents in which grid current is permitted to flow during a fraction 5 of a cycle.

Among amplifiers of this general class, the so-called class C amplifier, in particular, finds application as a plate modulated power amplifier. By definition, a class C amplifier is one in which plate current is allowed to fiow only during a fraction of each alternate half cycle of applied grid potential, that is, one in which the grid is biased beyond cut-01f. For efficient operation the grid is also allowed to be driven positive during a small fraction of each positive half cycle and the amplifier consequently draws grid current during this interval.

In the operation of a plate modulated class C amplifier circuit it is known that greater linearity of modulation is attainable if the grid bias potential of the electron discharge amplifier is permitted to vary at modulation frequencies. This suggests the desirability of employing a self bias impedance in the grid circuit, the potential across which can vary at modulation frequencies, rather than a separate source of fixed bias potential. However, if self bias alone is employed, failure of the carrier frequency excitation voltage results 30 in complete loss of grid bias and the consequent excessive plate current drawn by the discharge device may seriously damage it or even completely destroy it. Therefore, it has generally been considered advisable to employ suflicient fixed bias in addition to the self bias to protect the discharge device.

It is a primary object of my invention to provide means which normally permits efficient operation of an electron discharge amplifier circuit, with self bias alone if desired, and which is rendered operative upon reduction or interruption of excitation voltage to supply a bias to the amplifier to prevent damage thereto.

It is a further object of my invention to pro- D vide a supplemental source of bias potential which is normally rendered inoperative by the self bias developed across an impedance in the grid circuit of a class C amplifier and which automatically becomes operative to maintain the grid bias at a safe value in the event of failure of the self bias.

Still another object of my invention is to provide a plate modulated class C amplifier which is normally self biased by a potential varying at modulation frequencies and which is automatically provided with a protective fixed bias in the event of reduction of the self bias below a predetermined value.

The featuresof my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing which diagrammatically illustrates a plate modulated class C amplifier circuit embodying my invention.

Referring nowto the drawing, an electron discharge amplifier l is illustrated, having an anode or plate 2, a grid 3, and an indirectly heated cathode 4. The output circuit for the device comprises the conventional tuned circuit 5 coupled to the tuned circuit 6. The anode 2 is shunt fed from a source of potential I through the impedance 8 and the radio frequency choke 9 and the output circuit is isolated from the direct current source by the blocking capacitor I ll. Potentials of modulation frequency are impressed upon the impedance 8 through the blocking capacitor II from any suitable modulation source [2 which may, for example, comprise a source of audio frequency signals. The output of the device I is thereby plate modulated in a manner well known to those skilled in the art.

The input circuit for the device I comprises any suitable means for coupling an excitation voltage to the grid, illustrated as the coupling impedance I3, and a self bias impedance, illustrated as a grid leak [4, across which a self bias potential for the grid is developed by flow of grid current therethrough. The grid leak I4 is by-passed for voltages of excitation frequency by the capacitor l5. The excitation voltage is coupled to the impedance I3 from any suitable source, illustrated conventionally as the carrier voltage source l6, through the blocking capacitor ll.

Part or all of the grid leak M is also included in circuit with a supplemental source of bias potential. This is illustrated as a rectifier circuit comprising a full wave rectifier it, having a pair of anodes l9 and an indirectly heated cathode 20, and a suitable alternating current source, shown as the transformer 24. The rectifier output is applied, through a filter comprising the inductances 22 and 23 and the capacitor 26, across a selectable portion of the grid leak M in such a manner that flow of current in this circuit through grid leak I4 will render the grid 3 more negative with respect to the cathode 4. The inductance 22 performs the additional function of isolating the rectifier filter capacitor from currents of carrier or modulation frequencies in.the grid circuit.

For reasons that Will be apparent shortly, the potential impressed on rectifier l8 from sou ce 2| is proportioned so as to be less than the self bias potential normally developed across that portion of grid leak I4 included in the rectifier circuit. The relative magnitudes of these potentials may be adjusted either by varying the potential of source 2| or by varying the position of tap 25 on grid leak l4. Tap 25 also provides a means for adjusting the resistance load offered to rectifier [8 when operative.

Normally the self bias developed across the grid leak l4 by grid rectification of current supplied from the carrier voltage source 16 is sufficient to permit the device I to operate as a class C amplifier and the portion applied to rectifier [8 at all times exceeds the voltage impressed upon the rectifier l8 from the supply source 2|. It will be observed that this self bias potential appearing across the grid leak I4 is applied to the rectifier l8 in such direction that the anodes 19 thereof are rendered negative with respect to the cathode 20. 'I-Ience the rectifier I8 is normally nonconductive.

As the potential of the anode 2 fluctuates at the modulation frequencies supplied by the source It, the self bias potential across the grid leak l4 will likewise fluctuate, as will be apparent to those skilled in the art. As previously pointed out, this contributes to greater linearity of modulation. In some cases it may also be desirable to maintain a portion of the self bias at a conself bias across grid leak 14 decreases or disappears. When the potential applied to rectifier l8 from source 2| exceeds that applied thereto from grid leak I4, rectifier I8 is instantly rendered operative and produces a flow of rectifier current through grid leak M in the proper direction to maintain a negative bias on the grid 3. Thus,

excessive flow of plate current which might damage the device I is prevented.

Since the auxiliary rectifier circuit is'inoperative during the normal operation of the amplifier l and supplies a protective bias only during a non-operative condition of the amplifier I, the output need not be carefully filtered. Therefore, the inductancesZZ and 23 and the filter capacitor 24 may be relatively small and inexpensive. Thus, it Will be seen that I have provided a highly efficient amplifier circuit which is protected in a simple and economical manner, Although the amplifier I has been illustrated as operating normally with a self-bias alone, it will of course be obvious that additional fixed bias may be applied to the grid circuit which is effective during the operative condition of the amplifier.

While I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since modifications may be made, both in the circuit arrangement and instrumentalities employed, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention,

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In combination with an electron discharge amplifying device having an input grid circuit, means coupling an excitation voltage to said grid circuit to produce a flow of current therein, means toderive a bias for said grid from said current flow comprising a self bias impedance in said circuit, and normally inoperative biasing means rendered operative in response to reduction of said excitation voltage below a predetermined value to apply a negative bias to said grid.

2. In combination with an electron discharge amplifying device having an input grid circuit, means coupling an excitation voltage to said grid circuit to produce a flow of current therein, means to derive a self bias for said grid from said current flow comprising a self bias impedance in said circuit, a supplemental source of bias potential for said grid, and means normally rendered inoperative by said self bias potential and responsive to reduction of said self bias potential below a predetermined value to connect said source to said grid circuit.

3. In combination, an electron discharge amplifier having an input grid circuit, means coupling a source of excitation voltage to said circuit to produce current fiow therein, means to develop a self bias potential for said grid from said current fiow comprising a self bias impedance in said circuit, a supplemental source of bias potential for said grid, and means responsive to reduction of said self bias potential below a predetermined value to apply said supplemental bias to said grid, said last-named means including a rectifying device interconnecting said supplemental source and said impedance and normally maintained non-conductive by said self bias potential.

4. In combination, an electron discharge amplifier having a grid circuit and an output circuit, means coupling an excitation voltage to said grid circuit which is normally effective to produce current fiow in both of said circuits, means utilizing said grid current flow to produce a grid bias comprising a self bias impedance in said grid circuit, and means to apply a protective negative bias of lesser magnitude to said grid in response to reduction of said self bias below a predetermined value, said last-named means comprising a rectifying device and a supplemental potential source serially interconnected with said impedance, the potential applied to said rectifying device by said source being proportioned less than the potential normally applied thereto from said self bias impedance.

5. In a class C amplifier circuit, in combination, a thermionic amplifier having an input grid circuit, a high frequency excitation source coupled to said grid circuit, a grid resistor in said grid circuit across which a self bias potential for class C operation is developed by flow of rectified grid current during normal operation of said amplifier, a rectifier circuit including at least a portion of said resistor and which, when operative, produces additional rectified current flow in said resistor in the same direction, said rectifier circuit comprising a thermionic rectifier and an alternating potential supply source, and means to maintain the potential impressed on said rectifier from said source less than the portion of said grid leak self bias potential normally impressed thereon, whereby' said rectifier is nonconductive during normal operation of said amplifier.

6. In a plate modulated class C amplifier circuit, in combination, a thermionic amplifier having an input grid circuit and an output plate circuit, means normally impressing a carrier frequency excitation voltage on said grid circuit, a grid leak shunted by a carrier frequency b-y-pass capacitor in said grid circuit across which a self bias potential for class C operation is normally developed by grid current rectification, a source of modulation frequencies, means to vary-the current in said plate circuit in accordance with said modulation frequencies, a thermionic rectifier and an alternating potential supply source serially interconnected with at least a portion of said grid leak and efiective, when operative, to develop an additional protective negative bias for said grid across said portion, means to maintain the potential impressed on said rectifier from said source less than the portion of said grid leak self bias potential normally impressed thereon, whereby said rectifier is inoperative during normal operation of said amplifier, and means permitting the self bias potential across at least a portion of said grid leak to vary at modulation frequencies.

HENRY P. THOMAS. 

